The present invention related to medical imaging and, more particularly, to systems and methods for non-contrast enhanced pulmonary vein MR imaging, for example, using magnetic resonance imaging (MRI).
In MRI, when a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the nuclei in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) that is in the x-y plane and that is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mxy. A signal is emitted by the excited nuclei or “spins”, after the excitation signal B1 is terminated, and this signal may be received and processed to form an image.
When utilizing these “MR” signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received MR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The measurement cycle used to acquire each MR signal is performed under the direction of a pulse sequence produced by a pulse sequencer. Clinically available MRI systems store a library of such pulse sequences that can be prescribed to meet the needs of many different clinical applications. Research MRI systems include a library of clinically proven pulse sequences and they also enable the development of new pulse sequences.
The MR signals acquired with an MRI system are signal samples of the subject of the examination in Fourier space, or what is often referred to in the art as “k-space.” Each MR measurement cycle, or pulse sequence, typically samples a portion of k-space along a sampling trajectory characteristic of that pulse sequence. Most pulse sequences sample k-space in a raster scan-like pattern sometimes referred to as a “spin-warp”, a “Fourier”, a “rectilinear”, or a “Cartesian” scan. The spin-warp scan technique employs a variable amplitude phase encoding magnetic field gradient pulse prior to the acquisition of MR spin-echo signals to phase encode spatial information in the direction of this gradient. In a two-dimensional implementation (“2DFT”), for example, spatial information is encoded in one direction by applying a phase encoding gradient, Gy, along that direction, and then a spin-echo signal is acquired in the presence of a readout magnetic field gradient, Gx, in a direction orthogonal to the phase encoding direction. The readout gradient present during the spin-echo acquisition encodes spatial information in the orthogonal direction. In a typical 2DFT pulse sequence, the magnitude of the phase encoding gradient pulse, Gy, is incremented, ΔGy, in the sequence of measurement cycles, or “views” that are acquired during the scan to produce a set of k-space MR data from which an entire image can be reconstructed.
Atrial fibrillation is one of the most common sustained cardiac arrhythmias, afflicting over 2.2 million Americans and responsible for approximately one-third of arrhythmic hospitalizations. Following the recognition that ectopic beats in the pulmonary veins are a source of atrial fibrillation, pulmonary vein isolation (PVI) using radiofrequency (RF) ablation has become an accepted treatment. Imaging, such as using MRI, is commonly performed prior to the RF ablation treatment in order to identify the anatomic features of the pulmonary vein and left atrium and to assist procedural planning. Exemplary anatomic features include the number of pulmonary veins, pulmonary vein ostia size and orientation. Post-ablation pulmonary vein imaging is also conventionally performed in order to detect post-procedural complications, such as pulmonary vein stenosis.
Both multi-detector computed tomography (CT) and MRI are commonly used to image the pulmonary vein and left atrium; however, MRI offers the advantage of not exposing the patient to ionizing radiation or iodinated contrast. In current clinical practice, contrast enhanced MR angiography is conventionally used to perform imaging of the left atrium and pulmonary veins with a non-ECG gated spoiled gradient echo (GRE) imaging sequence.
Magnetic resonance angiography (MRA) uses the magnetic resonance phenomenon to produce images of the human vasculature. To enhance the diagnostic capability of MRA a contrast agent, such as gadolinium, is often injected into the patient prior to the MRA scan. Sampling of the central lines of k-space during peak arterial enhancement is key to the success of a CE MRA exam. If the central lines of k-space are acquired prior to the arrival of contrast, severe image artifacts can limit the diagnostic information in the image. Moreover, this data acquisition typically occurs during a prolonged breath-hold by the patient, which poses difficulties for those patients with atrial fibrillation. Since the pulmonary veins and left atrium are in close proximity to the right atrium, and great vessels such as the aorta, pulmonary artery, and superior and inferior vena cava, in the absence of contrast media, lack of contrast between the pulmonary vein and these adjacent structures is commonly observed. This lack of image contrast compromises pulmonary vein conspicuity. Data acquisition without ECG gating also poses difficulties for imaging the pulmonary veins, since doing so results in image blurring and over-estimation of pulmonary vein size. With the recent recognition of the association of nephrogenic systemic fibrosis (NSF) and gadolinium-based contrast media in patients with renal impairment, there has been increased interest in non-contrast enhanced MRA techniques.
One such non-contrast enhanced MRA technique is pulsed arterial spin labeling (ASL), which has been used in coronary, renal, and carotid artery MRA. In these methods, a slice selective inversion pulse is commonly applied proximal to the vessel of interest to label the in-flowing spins. After an inversion time (TI), during which the labeled spins flow into imaging slab, imaging is performed. To enhance the contrast, imaging is typically performed twice; once with and once without the labeling inversion pulse. A subtraction of the two data sets provides an angiogram with greatly suppressed stationary background tissue. In subtracting these data sets, however, pulsed ASL methods can introduce phase cancellations and produce signal-to-noise ratio (SNR) loss. To eliminate the need for subtraction, which also doubles imaging time, a modified double-inversion sequence has been proposed for coronary artery MRI, in which a non-selective inversion pulse is immediately followed by a two-dimensional selective pulse that locally re-inverts the ascending aorta. Aside from the undesirable increase in imaging time required for such double-inversion methods, lack of conspicuity between pulmonary veins and the left atrium and their adjacent structures is still prevalent in pulsed ASL methods, which prevents these methods from widespread clinical use.
It would therefore be desirable to provide a system and method for imaging of the pulmonary vein that does not rely on the use of an exogenous contrast agent or undesired ionizing radiation and that can produce images marked by significant contrast-to-noise ratio (CNR) between the pulmonary veins and adjacent anatomical structures. Moreover, it would be desirable to provide such a method in which breath-holding is not required by the subject under examination.